The sulfur content of heavy crude oil varies from 0.1 to 15 percent. This is mostly in the form of high molecular weight organic sulfur compounds, and any dissolved elemental sulfur and/or hydrogen sulfide represent only a small part of the total sulfur. The sulfur-containing compounds in crude oil include the following: sulfides, disulfides, mercaptans (thiophenes), benzothiophenes, dibenzothiopenes, benzonaphthothiophenes, and dinaphthothiophenes. The structures of these compounds are well known. The desulfurization of crude oil is an important preliminary step to improve the quality and yield of gasoline products. The current methods of desulfurization utilized in the chemical industry have fundamental limitations, such as the cost of energy and material consumption, severe processing conditions and the use of expensive catalysts. Processes that include microwave irradiation have also been disclosed in the prior art.
A principal purpose of hydrodesulfurization (HDS) is to improve the quality of the heavy crude oil to meet the required specifications for its particular use. Depending on the process conditions, the HDS process can be classified as “destructive” or “non-destructive”. The destructive HDS process is characterized by molecular fragmentation and hydrogenation saturation of the fragments to produce lower boiling fractions, and the non-destructive HDS process requires milder conditions, generally referred to as hydrotreating, and provides a means of removing simple sulfur compounds.
The effect of the HDS process is to convert the organic sulfur in the heavy crude to hydrogen sulfide as illustrated below:Heavy crudesulfur+H2→H2S+Heavy crudesulfur deficient 
This reaction is characterized by destructive hydrogenation which requires carbon-carbon bond cleavage and subsequent hydrogen saturation of the fragments leading to improved product quality through hydrodesulfurization and production of lower boiling point products. The process conditions require high temperatures and pressures, a catalyst and high hydrogen-to-crude oil feed ratios.
Application of radiation chemistry in the oil industry gained prominence in the early 1960's when only light hydrocarbon substances were used as models in radiation processing experiments. Radiation was deemed to be relatively expensive then and it was not until the 1990's that the technology referred to as hydrocarbon enhancement electron-beam technology (HEET) was developed. More recently, microwave irradiation has been used in the petroleum industry for inspecting coiled tubing and line pipe, measuring multiphase flow, and the mobilization of asphaltic crude oil. Gunal and Islam observed the permanent alteration of asphaltenes in the colloidal structures of the molecules and an increase in viscosity when exposed to microwave irradiation, due to the reorientation of molecular structures rather than thermal breakdown. “Alteration of Asphaltic Crude Rheology with Electromagnetic and Ultrasound Irradiation,” Journal of Petroleum Science and Engineering 2000, 26, 263-272. It was noted that when exposed to electromagnetic radiation, the presence of alsphaltenes caused permanent changes in crude oil rheology due to the polar nature of asphaltene molecules. Zaykin, et al. reported evidence of extensive branching and breaking of the paraffin chain during irradiation of paraffinic oil. “Radiation Thermal Conversion of Paraffinic Oil,” Radiation Physics and Chemistry, 2004, 69, 229-238; “Prospects for Irradiation Processing in the Petroleum Industry,” Radiation Physics and Chemistry, 2002, 63, 617-620.
In the microwave irradiation process, it is difficult to meet the requirements of the HDS destructive process due to its low energy, particularly in the absence of sensitizers. The prevailing conditions in the microwave process generally favor non-destructive HDS due to the low temperature conditions obtainable with microwave irradiation. Since crude oil absorbs little microwave radiation, sensitizers and other polar solvents can be used to improve its absorption. In the case of a water in crude oil emulsion, the retained water in the crude oil functions as the primary microwave energy absorber.
As a method of desulfurization, microwave heating has been recognized as providing advantages such as short start-up time, rapid heating, energy efficiency and precise process control. Through the use of microwave energy with additives, hydrocarbons high in sulfur content and/or composed of primarily heavy hydrocarbons can be made into useful commercial products which can be burned cleanly and efficiently as a fuel oil, as described in the following patents that disclose the use of microwave irradiation: U.S. Pat. No. 4,148,614, Apr. 10, 1979; U.S. Pat. No. 4,749,470, Jun. 7, 1988; U.S. Pat. No. 6,824,746; and U.S. Pat. No. 4,279,722, Nov. 15, 1994.
The use of microwave energy to demulsify otherwise hard to break emulsions of oil and water is also known to the art. These emulsions are commonly produced from the wells and must be removed and broken before the crude oil stream can be further processed. In some of the emulsions, the water is very tightly bound and the process for its removal is costly. Chemical demulsifiers are commonly used, but add an additional cost to the recovered oil and their presence can interfere with downstream processes. These agents are typically hydrophilic surfactants and synthetic or natural flocculants. Examples are quaternary ammonium siloxanes, tannin, sodium silicate, sodium pentahydate, and high molecular weight amines, acrylamies, acrylic acids, acrylates, and acrylate salts.
The terms crude oil-water emulsion and crude oil emulsion are used for convenience in the following description and in the claims to mean a water-in-crude oil emulsion.
It is therefore an object of the present invention to provide an efficient and practical desulfurization process that is integrated with the demulsification of the crude oil.
It is a further object of the invention to provide a catalytic process for hydrodesulfurization that is promoted by microwave energy under relatively mild conditions of temperature and pressure.